1. Field of the Invention
The present invention relates to a power-assisted bicycle including a power unit mounted on a body frame.
2. Description of the Related Art
A conventional power-assisted bicycle is disclosed in, e.g., Japanese Patent Laid-Open No. 2001-106163 (simply referred to as JP 2001-106163 hereinafter). This power-assisted bicycle includes a body frame that supports a front wheel and rear wheel, and a power unit mounted on the body frame. The power unit includes, for example, a pedal crank shaft which is rotated by the pedal force of a biker, a motor, and an output shaft. The resultant force of the pedal force (human power) of a biker and a motor driving force proportional to the magnitude of the pedal force is applied to the output shaft. The rotation of the output shaft is transmitted to the rear wheel via a chain.
The body frame includes, e.g., a head pipe which steerably supports a front fork, a down tube which extends rearward and downward from the head pipe, and a seat tube which extends upward from the lower end of the down tube. The power unit is mounted on a connector between the down tube and the seat tube via a plurality of power unit mounting brackets. A battery is installed above the power unit. The battery supplies power to the motor of the power unit. The battery has a shape which is elongated in an upper-lower direction and arranged between the seat tube and rear wheel.
The power unit mounting brackets are arranged in positions corresponding to the two ends of the power unit in the front-rear direction of the body frame, and in a position corresponding to the upper end of the power unit. Also, the pair of brackets each include plate-shaped portions extending in the left-right direction of the body frame. Each plate-shaped portion extends in the front-rear direction and in the upper-lower direction of the body frame, and includes a through hole to insert a fixing bolt. In addition, the plate-shaped portions are spaced apart from each other at an interval into which a mount of the power unit is inserted. The mount of the power unit includes a through hole to insert the fixing bolt, and is fastened to the plate-shaped portions by the fixing bolt while being inserted between the pair of plate-shaped portions. Therefore, the pair of plate-shaped portions elastically deform as they are pressed by the head of the fixing bolt and a nut, and are in tight contact with the mount as they are pushed from the two sides in the left-right direction.
In bicycles, including a power-assisted bicycle, the pedal force of a biker is desirably fully transmitted to the rear wheel.
To fully transmit the pedal force to the rear wheel in the power-assisted bicycle, the power unit must be mounted on the body frame in a rigid state in which the power unit cannot be displaced. To eliminate the displacement of the power unit, the rigidity of the power unit mounting brackets must be increased.
Unfortunately, the mounting structure of the power unit disclosed in JP 2001-106163 has the problem that the rigidity of the bracket that supports the power unit cannot further be increased. This is because, if the rigidity of the bracket increases, the plate-shaped portion hardly elastically deforms, so the pair of plate-shaped portions cannot clamp the mount of the power unit. When the plate-shaped portions and mount are made with high accuracy and the gap between them is minimized, the plate-shaped portions clamp the mount even if the elastic deformation amount of each plate-shaped portion is very small. However, this arrangement, which increases the accuracy more than necessary, cannot be used because the manufacturing cost unnecessarily increases.